(3br) Characterization of Fractal-Like Aerosols During Sintering

Characterization
of Fractal-like Aerosols during Sintering

M.L. Eggersdorfer

Particle
Technology Laboratory, Institute of Process Engineering, Department of
Mechanical and Process Engineering, ETH Zurich, Sonneggstrasse 3, CH-8092
Zürich, Switzerland.

Nanoparticle production in the gas phase
is a well-established route for carbon black, pigmentary titania and fumed
silica and a promising method for many new, sophisticated materials like
catalysts, sensors, phosphors, biomaterials and even nutritional products. Such
particles grow by gas and surface reaction, coagulation and sintering and tend
to form fractal-like aggregates and agglomerates affecting their transport,
light scattering, effective surface area and density [1]. The (real-time)
characterization of these structures and their constituent primary particles is
necessary for continuous monitoring of aerosol manufacturing and airborne pollutant
particle concentrations. Significant advances have been made in
characterization of agglomerates (physically ?bonded particles) by employing
fractal theory and relating agglomerate structure to its generation pattern
through the fractal dimension, D_f. What might have been
overlooked in characterization and simulations of fractal-like particles is
that the above D_f values have been developed for agglomerates
of monodisperse primary particles. For coagulating aerosols, however,
this needs to be carefully examined as Brownian coagulation leads to polydisperse
particles [2]. Furthermore, once coalescence or sintering starts between
these primary particles, sinter necks are formed between them converting the
agglomerates to aggregates [3]. Accounting for primary particle polydispersity
is important as the characteristic sintering time depends strongly on primary
particle size [4]. These properties may also affect their health impact
[5], e.g. agglomerates may undergo restructuring & break-up [6] and
release constituent primary particles.

                   The research during
my PhD focused on the formation of aggregates (chemically- or
sinter-bonded particles) by viscous flow sintering of amorphous materials
(silica, polymers) [3] and grain boundary diffusion sintering of crystalline
ceramics (titania, alumina) or metals (Ni, Fe, Ag etc.) [7] by multiparticle
sintering simulations. A scaling law was discovered between average aggregate
projected area and equivalent number of constituent primary particles during sintering:
from fractal-like agglomerates to aggregates and eventually compact particles.
This is a relation essentially independent of time, material properties and
sintering mechanisms. So the surface area mean primary particle
diameter was determined by (on-line) differential mobility analyzer (DMA) and
aerosol particle mass (APM) analyzer measurements and this power law for
aggregates. This primary particle diameter obtained during particle synthesis
is in good agreement with ex-situ measurements like nitrogen adsorption and
particle counts from microscopic images.

[1] P. Meakin, Fractal
aggregates, Adv. Colloid Interface Sci. 28 (1988) 249.

[2] M.L.
Eggersdorfer, S.E. Pratsinis, The structure of agglomerates consisting of
polydisperse particles, Aerosol Sci. Technol. 46 (2012) 347.

[3] M.L.
Eggersdorfer, D. Kadau, H.J. Herrmann, S.E. Pratsinis, Multiparticle sintering
dynamics: from fractal-like aggregates to compact structures, Langmuir
27 (2011) 6358.

[4] M. Sander,
R.H. West, M.S. Celnik, M. Kraft, A detailed model for the sintering of
polydispersed nanoparticle agglomerates, Aerosol Sci. Technol. 43 (2009)
978.

[5] L.K.
Limbach, P. Wick, P. Manser, R.N. Grass, A. Bruinink, W.J. Stark, Exposure of
engineered nanoparticles to human lung epithelial cells: Influence of chemical
composition and catalytic activity on oxidative stress, Environ. Sci.
Technol. 41 (2007) 4158.

[6] M.L.
Eggersdorfer, D. Kadau, H.J. Herrmann, S.E. Pratsinis, Fragmentation and
restructuring of soft-agglomerates under shear, J. Colloid Interface Sci.
342 (2010) 261.

[7] M.L.
Eggersdorfer, D. Kadau, H.J. Herrmann, S.E. Pratsinis, Aggregate morphology
evolution by sintering: Number & diameter of primary particles, J.
Aerosol Sci. 46 (2012) 7.